Switching devices protect electrical power supply systems and loads in the event of a short circuit by rapidly building up a sufficiently high switching voltage. As a result of this, the short-circuit current is limited, and is interrupted after a short time. In order to increase the current limiting effect, the switching voltage that is used can be increased by connecting the switching device in series with a separate current limiter. According to the prior art, the limiter is for this purpose connected in the main circuit, so that the load current flows through it all the time, both during normal operation and in the event of a short circuit.
There are various technical solutions for limiters. In addition to conventional mechanical switches which produce switching arcs, PTC limiters are used for current limiting, in which the build up of voltage when switching occurs is produced by increasing the electrical resistance of the limiter material and/or by way of a gas discharge with a high burning voltage.
In comparison to mechanical switches, PTC limiters have the advantage that the switching voltage is built up very quickly. The disadvantage is the greater cold electrical resistance. As a result of this, the rated current must be limited during rated operation in order to prevent unacceptable heating of the PTC material, for example as a result of motor starting currents, and unintentional response of the limiter. In one known commercial product (ABB PROLIM), a circuit breaker, acting as a switching device, and a PTC limiter are specifically electrically connected in series in the main circuit.
Another possible solution for the rated current problem is described in EP 0 657 062 B1, in which a limiter for a circuit breaker is connected in an auxiliary current path, through which current flows only briefly, when switching occurs. The auxiliary current path is formed from the arc guide rails and the quenching chamber, and is connected by the commutation of the arc from the switching contacts onto the guide rails.
In comparison to conventional circuits in which a circuit breaker and a limiter are connected in series, there is, however, a risk of switching failure, with the switching arc possibly being commutated back to the main current path, when the limiter is connected in the auxiliary current path. When commutation back such as this occurs, the switching arc or the arc attachment point is moved back from the auxiliary current path to the main current path, for example to the switching contacts. As a result of this, although no current passes through the limiter, it does, however, generally retain its resistance at that time. If another attempt is made for the arc to commutate into the switching chamber, the limiter switching voltage must then be overcome in addition which, in some circumstances, makes the commutation process so difficult that it can fail.
DE 42 43 314 A discloses a current-limiting circuit breaker with an arc quenching device and an auxiliary current path with at least one PTC thermistor and an overvoltage suppressor associated with it. In both devices, the switching from the current-limiting mode to the non-current-limiting mode takes place in a corresponding manner to the overcurrent decay. Furthermore, U.S. Pat. No. 5,777,286 discloses an electrical switching device with separate contacts, which can be disconnected mechanically, and with an arc switching contact associated with this, in which case a PTC thermistor or the like can be connected in the auxiliary circuit. Furthermore, EP 0 350 825 A2 discloses an electrical switching device with an arc quenching device and a current limiting device in the auxiliary circuit.